General comments

Chen et al. review the methods for estimating background ozone concentrations, which determine the maximum achievable reduction in ozone pollution through mitigation of local anthropogenic emissions, with a specific focus on background ozone in China. The paper is informative and organized, but it is overly long in some sections. It is also important for the authors to say why this review is needed when there are so many other similar recent papers reviewing ozone in China (Liu et al., 2020; Lu et al., 2019; Sahu et al., 2021; Wang et al., 2022; Xu, 2021). I think the key is that this paper analyzes background ozone specifically, but that concept is also addressed quite thoroughly in most papers reviewing drivers of ozone concentrations and trends.

I would also suggest that the authors significantly reduce the length of Section 4, which is quite long-winded and repetitive. Section 5.2 and Figure 5 are the most interesting pieces of the review, and the discussion seemed too short. Why is the integrated method a much smaller range? Is there regional variability in the background ozone estimated from the integrated method? Should we “trust” the results of the integrated method above the other methods and, if so, why? I think this information is discussed more in Section 4.4, but I would move this information to the section where Figure 5 is discussed.

In addition, I would expect the paper to have more discussion of the seasonality of background ozone, which is significant in a region like China (Chen et al., 2023). Finally, I suggest adding the following citations from potentially relevant studies. Han et al. (2019) quantify the influence of foreign ozone on East Asia using GEOS-Chem simulations. Colombi et al. (2023) found that subsidence of elevated free troposphere ozone drove surface background ozone in East Asia in May to exceed 50 ppb, which is much higher than the background values in North America and Europe. Wang et al. (2022) showed that the high free tropospheric ozone in East Asia has been increasing in the last two decades, which may lead to increased background ozone. Ma et al. (2025) diagnose causes of high free tropospheric ozone. Luo et al. (2024) also discuss STE as a source of background ozone in the North China Plain. There may be many other papers I am missing, but I leave the decision of whether to add these citations up to the authors’ discretion.

Overall, I think this review is potentially useful, but would benefit from shortening Section 4 and more clearly stating why this review is different and necessary given the many other reviews of ozone in China from the last 5 years.

Specific comments

• I suggest adding a sentence that clearly describes the concept of background ozone to the Introduction of the paper? For example, around line 68 in the introduction, I think a sentence is needed that says something like: “Background surface ozone is defined as the ozone that would be present in a given region in the absence of local anthropogenic emissions, and can be influenced by local natural factors or distant anthropogenic and natural factors.” It is stated in the abstract, but should be defined again in the introduction.
• Figure 1 is a nice visualization. I like how the beaker implies chemical reactions occurring in the atmosphere, and shows how anthropogenic emissions add ozone on top of the background.
• Line 163: The conversion from µg L-3 to ppb is strange to me. Why would you assume a temperature of 0˚C? How big of a difference does it make if you assume a more reasonable temperature and pressure rather than STP? Could you use meteorological data to assume a more accurate and precise temperature and pressure?
• Figure 3 is pretty wordy and not easy to absorb the information presented. I suggest significantly revising this figure to make it easier to digest and more visually appealing. At the very least, the authors should remove unnecessary words (e.g., "suitable" is over-used).
• Figure 4: Could you please make the shading of each region be a color that represents the magnitude of the background ozone concentration? For example, a color bar that ranges from 25 to 50 ppb where lower background ozone is a lighter color and higher background is a darker color. It is hard to view the spatial variability of background ozone from the bar charts.
• Figure 5: where are the estimates of background ozone from numerical modeling, statistical methods, and integrated methods coming from? I assume it is from Table S1, but there is no citation or reference to table S1 in figure caption. Is n= at the bottom of each box referring to the number of studies compiled or the number of measurements? Overall, this caption should be more descriptive.
• Section 5.3 and Figure 6: Why is NC the only region that has two separate lines showing the trends in background ozone?
• Same comment for Figure 7 as made for Figure 4. Please consider shading the regions according to the background ozone concentration
• The conclusions section is informative and written well

Technical corrections

• Lines 8-10: The first sentence of the abstract is worded in a confusing way. I suggest: Background ozone (O3) refers to baseline O3 concentrations in the absence of local anthropogenic emissions, critical for understanding and mitigating tropospheric O3 pollution.

Citations:

Chen, Z., Xie, Y., Liu, J., Shen, L., Cheng, X., Han, H., et al. (2023). Distinct seasonality in vertical variations of tropospheric ozone over coastal regions of southern China. Science of The Total Environment, 874, 162423. https://doi.org/10.1016/j.scitotenv.2023.162423

Colombi, N. K., Jacob, D. J., Yang, L. H., Zhai, S., Shah, V., Grange, S. K., et al. (2023). Why is ozone in South Korea and the Seoul metropolitan area so high and increasing? Atmospheric Chemistry and Physics, 23(7), 4031–4044. https://doi.org/10.5194/acp-23-4031-2023

Han, H., Liu, J., Yuan, H., Wang, T., Zhuang, B., & Zhang, X. (2019). Foreign influences on tropospheric ozone over East Asia through global atmospheric transport. Atmospheric Chemistry and Physics, 19(19), 12495–12514. https://doi.org/10.5194/acp-19-12495-2019

Liu, H., Zhang, M., & Han, X. (2020). A review of surface ozone source apportionment in China. Atmospheric and Oceanic Science Letters, 13(5), 470–484. https://doi.org/10.1080/16742834.2020.1768025

Lu, X., Zhang, L., & Shen, L. (2019). Meteorology and Climate Influences on Tropospheric Ozone: a Review of Natural Sources, Chemistry, and Transport Patterns. Current Pollution Reports, 5(4), 238–260. https://doi.org/10.1007/s40726-019-00118-3

Luo, Y., Zhao, T., Meng, K., Hu, J., Yang, Q., Bai, Y., et al. (2024). A mechanism of stratospheric O ₃ intrusion into the atmospheric environment: a case study of the North China Plain. Atmospheric Chemistry and Physics, 24(12), 7013–7026. https://doi.org/10.5194/acp-24-7013-2024

Ma, X., Huang, J., Hegglin, M. I., Jöckel, P., & Zhao, T. (2025). Causes of growing middle-to-upper tropospheric ozone over the northwest Pacific region. Atmospheric Chemistry and Physics, 25(2), 943–958. https://doi.org/10.5194/acp-25-943-2025

Sahu, S. K., Liu, S., Liu, S., Ding, D., & Xing, J. (2021). Ozone pollution in China: Background and transboundary contributions to ozone concentration and related health effects across the country. Science of The Total Environment, 761, 144131. https://doi.org/10.1016/j.scitotenv.2020.144131

Wang, T., Xue, L., Feng, Z., Dai, J., Zhang, Y., & Tan, Y. (2022). Ground-level ozone pollution in China: a synthesis of recent findings on influencing factors and impacts. Environmental Research Letters, 17(6), 063003. https://doi.org/10.1088/1748-9326/ac69fe

Xu, X. (2021). Recent advances in studies of ozone pollution and impacts in China: A short review. Current Opinion in Environmental Science & Health, 19, 100225. https://doi.org/10.1016/j.coesh.2020.100225

Chen et al. present a comprehensive review of definitions and methods for estimating tropospheric background ozone concentration, with a specific focus on China. The manuscript is well-structured and demonstrates considerable effort in literature compilation. However, I have several major concerns that should be addressed before the manuscript can be considered for publication.

For Section 3, the authors discuss multiple definitions, including USBO, PRBO, RBO, and NBO. While the historical evolution is clearly present, the distinctions between these items may be difficult for readers to follow. I suggest adding a summary table outlining the key characteristics of each definition, such as name, included and excluded sources, and typical applications. Alternatively, the authors could consider incorporating these details into Figure 2.

In Section 4.2, the authors introduce several statistical methods estimating background O3. It would be helpful if the authors could compare the results from these different methods at the same location, using an example to illustrate their differences to highlight the respective advantages and disadvantages of each approach.

Also, the manuscript suggests that integrated methods improve the estimation of background O3 by combining various data sources and techniques in section 4.4. How it is unclear how these results compare quantitatively with those from individual methods discussed in the above sections. And how strong are we confident with these integrated results? Providing a concrete example or case study to illustrate the improvement would strengthen this section.

The meta-analysis is a key strength of this manuscript, but details on how the data were compiled and interpreted are limited. Clarifying the data processing criteria and handling of uncertainties would enhance this part of the work.

Minor:

Line 132: there is an unnecessary comma

Line 355: “causa technique” likely intended as “causal technique”